Thermal radiation of the normal temperature optical system coinciding with the target radiation spectrum is the main background noise source for long-distance infrared detection of weak targets. It will reduce the detection sensitivity and detection distance of the system and increase the difficulty of target detection and recognition. Reducing the temperature of the optical system is the most direct and effective way to reduce its own radiation, which can reduce the background noise of the system. The cooling time and temperature characteristics of the lens under different optical-mechanical structures are simulated. The simulation results show that the cryogenic lens assembly with copper material optical-mechanical structure has a heat leakage of 0.2W at 180K, and the temperature difference between the center points of the two lenses is 0.8K. A miniaturized ultra-high frequency pulse tube cryocooler is used as a cold source to cool the lens assembly of 30 g optical-mechanical thermal mass. The temperature characteristics of the lens under different input power of the cryocooler are tested. By optimizing the temperature control strategy, the lens temperature can be stabilized at 180 K in 15 minutes, the temperature fluctuation is ± 0.2K, and the temperature difference between the two lenses is less than 1K, which is a useful exploration for the infrared detection system directly integrated with cryogenic optics.
The Einstein Probe (EP) is an X-ray astronomical mission mainly devoting to time-domain astronomy. There are two main scientific payloads onboard EP, the Wide Field X-ray Telescope (WXT) based on the lobster eye optics and the Follow-up X-ray Telescope (FXT). FXT contains two Wolter-1 mirrors with a pnCCD detector on each focus. The total effective area is about 600 cm2 and the energy range is 0.3-10 keV. The pnCCD detector cooled by a pulse tube cooler enables high-resolution spectroscopy and imaging combined with excellent time resolution. It will also have several working modes with time resolution ranging from tens of microseconds to 50 milliseconds. Currently, the FXT is in its qualification model phase. The mirror assemblies (STM and TCM) as well as the pnCCD EM module have been manufactured and tested.
Integration of optics inside a detector-dewar-cooled-assembly (DDCA) is a good strategy to miniaturized infrared cameras in order to provide small payload systems with thermal vision capability for both military and civilian applications. The optical additional mass has to be very small in order to limit the cool-down time of the DDCA. However, reducing the mass of these optical systems results in a decrease of the resolution and impaction on the image quality, making them difficult to use in the high-performance and high sensitivity applications. In order to achieve better optical performances, a classical optical system which consisted of four lenses was integrated in the DDCA. By optimizing the heat transfer of the lens mount and using a miniature pulse tube cryocooler(MPTC) as the cooling source, the cooling time of the system was reduced. The lens mount and the lenses were cooled down to 80K in an hour by the MPTC with 45W input power. The total mass to be cooled is 100 g, where the mass of the lenses is 15 g. In the later prototypes, the mass of the lens mount can be greatly reduced by optimizing the installation of the detector and the lens mount, and the cooling time of the cryogenic camera integrated with the high-performance optical system can be reduced to an acceptable range by using the lens mount heat transfer optimization method described in this paper.
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